Every basic chord is a triad — three notes stacked in thirds. Change the size of those thirds and you get four flavours: major, minor, diminished and augmented. See them on the piano, then discover where the whole idea came from — a single string.
A triad is three notes: a root, a note a third above it, and one more third on top (the fifth). A "third" always spans two letter-names — but it comes in two sizes: a major third (4 semitones) and a minor third (3 semitones).
Which two sizes you stack decides the chord's quality. Stack a big third then a small one and you've got a major chord; swap the order and it's minor. Everything below is that one idea, shown three ways.
The keyboard lays every semitone out in a row, so a chord is easy to see. Count 4 semitones up for a major third, 3 for a minor third.
Long before pianos existed, musicians worked out how notes fit together by measuring a single stretched string. The ancient Greeks — Pythagoras among them — used a one-string instrument called the monochord, sliding a bridge along it and listening: halve the string and you hear the same note an octave higher; take two-thirds of it and you get a perfect fifth. Whole-number ratios, one string.
That monochord stayed the main teaching tool right through the Middle Ages. Theorists like Guido of Arezzo used it to work out scales and train church singers, and lute and viol players tied gut frets across the neck at those same distances. Each fret step is one semitone — so a fretted string is really a ruler for building intervals. Stack a few of those intervals and you have a chord.
Because one fret = one semitone, you can build a whole chord on a single string just by counting frets. Here is each chord climbing up the A string in C — root, third, fifth:
You wouldn't really play a chord all on one string — you spread the same notes across the neck. Same three notes, marked R · 3 · 5 wherever they fall. Pick your instrument:
| Chord | Lower third | Upper third | Fifth | Semitones |
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Tip: the little numbers over the diagrams are how many semitones to count up for each third — on the piano that's keys, on a string that's frets.